<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>后盾人</title>
  </head>
  <body>
    <input type="text" name="age" />
    <span id="msg"></span>

    <script>
      let span = document.querySelector("#msg");
      document
        .querySelector('[name="age"]')
        .addEventListener("keyup", function() {
          span.innerHTML = this.value >= 90 ? "年龄不能超过90岁" : "";
        });
      // let a = 1,
      //   b = 2,
      //   c = "1";
      // == ===  > < >= <=
      // console.log(a === c);
      // console.log(b >= 2);
      // console.log(b <= 2);
    </script>
  </body>
</html>
